1. Field of the Invention
The present invention relates to a wavefront aberration measuring apparatus, a wavefront aberration measuring method, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
In manufacturing semiconductor devices, such as IC and LSI, image pickup devices, such as a CCD sensor, display devices, such as a liquid crystal panel, and other devices, such as a magnetic head, with photolithography, a projection exposure apparatus is used to transfer a pattern, which is formed on a mask (reticle) as an original, onto an exposure target. The projection exposure apparatus is needed to accurately transfer the pattern on the reticle onto the exposure target at a predetermined magnification. It is, therefore, important that the exposure apparatus has high imaging performance and uses a projection optical system having reduced aberration.
Recently, with an increasing demand for further miniaturization of semiconductor devices, transfer patterns have become more sensitive to aberration of an optical system. For that reason, there is a demand for measuring optical performance, e.g., wavefront aberration, of the projection optical system with high accuracy. In addition, simplification, speedup, and cost reduction of the measurement are also important factors from the viewpoint of increasing productivity and economy.
A known method of actually printing a mask pattern on a wafer, i.e., a substrate, and observing a printed resist image by, e.g., a scanning electron microscope (SEM) for inspection takes a time to perform the inspection, including exposure, development, etc. Also, reproducibility of the inspection is poor due to errors caused in applying a resist and developing an image.
To overcome the above-described problems, measuring apparatuses have hitherto been proposed which are known as, e.g., a Point Diffraction Interferometer (PDI) having a pinhole to form an ideal spherical wave, and a Shearing Interferometer utilizing the shearing interference.
For example, Japanese Patent Laid-Open No. 2000-146705 and No. 2000-097666 propose a wavefront aberration measuring apparatus employing a Line Diffraction Interferometer (LDI) which has a slit for forming an ideal cylindrical wave or an ideal elliptic wave.
In the known LDI-type wavefront aberration measuring apparatus, a measurement mark in the form of a window-slit member is put on a plate arranged under a projection optical system, i.e., an optical system to be inspected.
The measurement mark includes a slit shape for forming an ideal wavefront and a window shape through which the wavefront containing aberration information of the inspected optical system passes. A two-dimensional photodetector is arranged just under the plate.
The wavefront aberration measurement of the inspected optical system is performed by taking an image of interference fringes of two wavefronts, which are generated by the window-slit member, with a two-dimensional photodetector, and by executing image processing of the taken image of the interference fringes. The image processing includes an image conversion process for converting data of the taken image of the interference fringes from coordinates on the light receiving surface to coordinates on the pupil plane of the projection optical system, and a wavefront aberration calculating process for extracting wavefront aberration information from the image data of the interference fringes after the image conversion.
If the state of the generated interference fringes taken as the image data is not satisfactory due to, for example, deviations of the various members from their optimum measurement positions and deposition of dust on the light receiving surface, this increases a probability that a false processing result is calculated or a measurement error is caused when the above-described image processing is executed.
In the related art, to prevent deterioration of measurement performance caused by the above-described factors, the positions of the various members are optimized while visually confirming the image of the interference fringes. Also, whether interference fringes are generated in such a state as enabling a fringe analysis process to be executed in practice is determined by confirming a phase distribution (or measured values), which is calculated through the image processing, with respect to the taken image data of the interference fringes.
However, with the technique of adjusting the measurement members to their optimum positions in a visual manner or based on design values, or with the technique of visually confirming the phase distribution (or measured values) calculated through the image processing, the interference fringes cannot be stably obtained in a satisfactory state, thus deteriorating the measurement accuracy and prolonging the measurement time.
The above discussion is applied to not only the known LDI-type wavefront aberration measuring apparatus, but also all conventional types of optical performance measuring apparatuses, including the PDI type and the shearing interference type, in which the optical performance of an optical system to be inspected is measured on the basis of two-dimensional information.